backends/drm: Make DrmOutput::queueChanges() not touch active status

The DrmOutput synchronizes the enabled state with the active state,
which makes sense on one hand, but on the other hand, that's not good.
The drm backend makes a decision that should be ideally made by either
kscreen (turn on outputs before applying an output config), user, or
kwin itself.

This would also allow kwin to control the allocation of crtcs for
non-desktop outputs, which is a minor thing, but it might be useful in
the future.